1. Field of the Invention
The present invention relates to a wheel axle bearing unit, and more particularly to a bearing unit that has an enlarged space inside the bearing, for preventing overheating and leakage of grease, as well as for making the unit lighter in weight.
2. Description of the Related Art
FIG. 3 shows a conventional wheel axle (non-driven) bearing unit 11. The bearing unit 11 is called a third generation bearing unit, which includes an outer race 12 constituting an outer member, provided with dual raceways namely outer raceways 12a, 12b along an inner circumferential surface thereof, a hub 13 provided with an outer-side inner raceway 13a along an outer circumferential surface thereof, an inner race 14 provided with an inner-side inner raceway 14a along an outer circumferential surface thereof and fitted to an outer circumferential surface of the hub, spherical rolling elements 15 aligned in dual rows between the outer raceways 12a, 12b of the outer race 12 and the inner raceways 13a, 14a of the hub 13 and the inner race 14, and retainers 16, 17 each of which retains the rolling elements 15 at regular intervals in a circumferential direction. The hub 13 and the inner race 14 are integrally formed, and serve as an inner member.
A body mounting flange 18 is provided on an outer circumferential surface of the outer race 12, and a wheel mounting flange 19 is provided on an outer circumferential surface of the hub. A wheel (not shown) is mounted to the wheel mounting flange 19 with a plurality of bolts 20. A short cylindrical portion 13b formed at an inner end portion of the hub 13 is plastically deformed in a radial direction, thus to be pressed against an end face of the inner race 14 with a predetermined pressure. A first seal 21 is disposed between the outer race 12 and the hub 13 at a position adjacent to an outer edge of the outer-side raceways 12a, 13a, and a second seal 22 is disposed between the outer race 12 and the inner race 14 at a position adjacent to an inner edge of the inner-side raceways 12b, 14a. 
An appropriate amount of grease having high lubricity is sealed inside the bearing (a portion constituted of the raceways and the rolling elements) of the wheel axle bearing unit 11. The first seal 21 and the second seal 22 serve to prevent leakage of the grease. The sealed grease contains for example a mineral oil as a base oil, to which a predetermined weight percent of thickening agent is added. A thickening agent currently in popular use is an urea-based grease corresponding to type 1 according to JIS K 2220 for high load, viscosity number 2.
Oil film of the grease on a contact face between the raceways 12a, 13a, 12b, 14a and the rolling elements 15 prevents occurrence of fretting, premature seizure and flaking. The oil film deteriorates with a load, temperature and so on, therefore it is desirable to constantly supply a trace of oil to the contact face between the raceways and the rolling elements during rotation. Accordingly, the grease is sealed in a central portion of the raceways and between the rolling elements, so as to fill approx. 50% of a space inside the bearing. Here, a thrust roller bearing is known, which has an enlarged interior space so as to retain more amount of grease (Ref. Japanese Laid Open Patent No. 2000-85306).
A difference between the present invention and the Japanese Laid Open Patent No. 2000-85306 is that the latter is designed such that the outer race rotates, and that a groove is provided along an inner circumferential surface of the outer member, in which the grease is supplied for lubricating the raceway and the rolling elements. However, it has proved that in a system where the inner race rotates a centrifugal force is not applied to the outer member, and that hence such design is not workable.
The present invention is based on the inner race rotation, and a first grooved portion is provided along an outer circumferential surface of the inner member, so that the grease sealed in the first grooved portion is distributed by a centrifugal force to the raceways and the rolling elements. Also, a second grooved portion formed along an inner circumferential surface of the outer member is intended for depositing therein a surplus of the grease supplied from the first grooved portion and, therefore, the second grooved portion may be omitted, provided that an appropriate amount of grease is to be supplied.
While it is desirable, as already stated, that a trace of oil is constantly supplied to the raceways 12a, 13a, 12b, 14a, actually the grease sealed between the raceways 12a, 13a and the raceways 12b, 14a (shadowed area in FIG. 3) constitutes a layered structure of a uniform thickness, which is prone to flow even with relatively weak vibration. Consequently, in the case where a large amount of grease accidentally flows into between the rolling elements 15, there may be caused overheating because of fluid friction of the grease itself, or softening or oil separation due to breakdown of fiber structure of a thickening agent (for retaining an oily substance) in the event that the thickening agent has a low mechanical shear stability, which may even lead to grease leakage.
For preventing the grease leakage, decreasing a sealing amount of the grease can be an option. Also, a grease having a higher viscosity may be employed, or a shear stability of the thickening agent may be upgraded, in order to prevent overheating. However, since such measures are related with a life span of the bearing, the examination and solution-making are too complicated to readily carry out.